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Essay: Nano Membranes

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A membrane can be defined as a selective barrier, it means that it can allow a few particles of liquid and block others. Membrane is use to separate certain particles from liquid. There are two types of particles in liquid; one that passes through membrane while the other is retained particle. The liquid that passes is called permeate while retained particles is called retentive or sometimes called concentrated. Here, we can achieve separation by selectively passing the necessary or required components of liquid while retain other components which are not required or harmful components. From fig 1.1 it is clearly show that how membrane block unwanted components and allow required components. Membranes have taken very important position in chemical field of technology and it is practically using in many application. It can separate on the basis of particle sizes and molecular weights.
The above figure shows the function of membrane. It is designed as; it will pass only small particles and will block large particles. So the left side of membrane is allowable particles while the right side particles are ready to separate. It works like a grill of bird cage which stops bird and do not allow a bird to go out but allows air to come in the cage. By using this we can separate a harmful or restrict particles from liquid and allow healthy particles and at the end we get a required liquid or particles. The main purpose of membranes is in production of drinking water and separation of harmful particles of industrial gases from water, it can be use in filtration of particles from water. you will amazed that membrane is using since eighteenth century but was used until the end of World War II. Membrane filter were used to test water safety level. There were few drawbacks that why it was not widely exploited. The drawbacks were lack of reliability, operation process was slow, selectivity reduced and high cost.
1. Polymer Nano membranes are Cellulose Acetate, Polycarbonate, PVC copolymer, Polyvinyledendi fluoride, Aromatic Polyamide, Cellulose Nitrate, Poly propylene, Poly acrylo nitrile, Poly tetra fluoro ethylene, Polyvinyl chloride, aliphatic polyamide, Poly di methyl silo xane etc.
2. Nano Membranes of inorganic substances are titanium, aluminum, silicium and zirconium oxides, also slilicium carbide. The others are metal based like zeolites, iron palladium and carbons etc.
3. Membranes having Nano functional polymer based membrane such as polysulfone which is doped with silver NP and cause bio chemical fouling resistant.
There are different properties of Nano membranes which defines the characteristics and quality of nano membranes. On the basis of property, we are able to use nano membranes for various functions.
1. It is an absolute barrier and it blocks particles which is need to be separate while pass the particles which are required for use. The barrier can be change on the basis of requirement.
2. The process of separation can be chosen according to the requirement of treatment.
3. There is no need to use chemical for disinfection process. Simply use nano membranes and it will separate disinfected particles from a substance by defining the pore size smaller than the disinfected particles and separate it from main substance.
4. The space required for treatment is less than conventional scheme because it has very simple function and just need to pass substance through nano membrane. All we need to do is; to have a nano membrane and a pressure use to pass the substance through nano membrane.
5. It has a modular design and has flexibility in the variety of use. Its design can be modified on the basis of function of use.
6. Fluxes
7. The requirement of energy for the process is low and need overall small energy. The energy required to generate pressure on the substance.
There are some limitations in nano membrane. But when we see the advantages of nano membranes then its limitations seem negligible. So let’s discuss the limitations of nano membrane and later we shall discuss the benefits and advantages of nano membrane and its application. The Limitations of nano membranes are actually its disadvantage which makes it less popular than conventional but in many conditions and to get high level result we compromise on it.
1. Fouling of membrane: Nano membrane cause fouling after using few time. It is the main problem, which make the process costly. It cause due to different reasons. The fouling of nano membrane may depend on operational change of the system; if the membrane is made for one operation condition and when it has to work on the change in operating condition which may be temperature, over pressure or anything; this cause the fouling of nano membrane. Other reasons on which it depends is optimization and feed composition.
2. Stability of membrane: There is another limitation in nano membranes which is stability. Nano membranes do not remain stable for so long. After using few times its stability decreases and it does not give good results as before. From this problem we can conclude that we need to change nano membranes after few times of using otherwise we would not be able to get high level results; and changing nano membrane after every periods of use will cause a great increase in cost. And the process will become not bearable with respect to the cost. The stability of nano membranes may depend on chemical resistance against the agents use for cleaning materials. When use nano membrane, there is a need to clean nano membranes after few times, the agent use to clean up the nano membranes can cause the stability problem of nano membrane.
It has a synthetic structure and has thickness less than 100 nm. There are different types of pore structure shapes; it may be Cylindrical, Pyramidal and Conical to make various type materials. Patterning is used for surface sculpting and modification of Nano membrane; it also includes the Nano arrays of hole. These dimensions make a hybrid between Nano and micro system, but the system remains in Nano meter scale. For high fluxes and selectivity; molecular design helps a lot at Nano meter scale like polymers. There are many applications in various fields in which it is using that are pharmaceutical, industrial, and medical including purification separation of protein, detection in bio molecule. The understanding of pore size effect and modification of shape pore surface is important, also the possible flow of osmosis and variability of electric field within a pores of Nano membrane. There are many features of membrane in Nano meter scale like electronics, optics, catalysis, purification and filtration, selective molecule separation etc.
Figure 1: Nano membrance structure built in three layers
It makes sandwich structures with different properties by depositing ultrathin layer. The each layer’s properties are not itself modified .The adding of new strata are still quasi 2D, its thickness is in nanometer range and smaller than its width and length. The Nano membranes of layered structure are a special case of laminar Nano composites. It can consist of two or more strata. The used stratum has its own functionalities and properties in composition and it contribute its properties in multi functionality purpose. These may be single crystal, homogeneous mixture Nano agglomerates, single crystals and poly crystals etc. It may include oxides and silicon, etc. These type of structures may be gradient and uniform but always homogenized near atomic / molecular at a dimension level of much smaller thickness than Nano membrane. Let take an example for instance, There are different layer of stratum one may have a chemical or biological activity, to attract selectively specific species use ligand layer; one may have plasmonic waveguide and ensure the possibility for sensing of biological behavior and another layer may be use to strengthen the mechanical properties of composite structure.
Figure 4: a) Nano triangular prism b) Nano ring c) Nano cubes
The above figure shows the possible Nano filters. First one is triangular Nano prism, the second one is Nano ring type filter; the same that is shown in above example and in fig 1, the third one is Nano cube filter, all names are defined by considering its physical shape. When it comes to the introduction of Nano particle filters; it must be performed under consideration of mechanical properties without compromising these properties.
The lamination is a process, in which material is chemically reacting with another chemical of resistive property; to cover the material with the protective layer of another chemical so that to increase the life of material. To protect and increase nano membranes life we need to make lamination over nano membranes so that it could be use for long. Let’s discuss the lamination of nano membrane in detail and study the types of lamination use for nano membrane protection. The minimum limit of specific stratum thickness is determines from a single monomolecular / monatomic layer. Laminations is considered as a super structure of nano membrane; because chemical is added by chemical reaction which makes layer over nano membrane and make a sandwich of chemicals. The super structure sandwich obtained are denoted as stratified, laminar, or layered. When the term super structure use in nano membranes it means the substance is laminated and layered with protective material. The Nano membranes of layered structure are a special case of laminar Nano composites. It can consist of two or more strata. The used stratum has its own functionalities and properties in composition and it contribute its properties in multi functionality purpose
1. Micro Fabrication: Micro fabrication is the basic way of fabricating laminar Nano membrane. In practical, all thin film deposition methods can be use to this purpose, the sensitivity of specific material defines its limit to the operating conditions. It is usually assumed that the most often used techniques are sputtering deposition of required combination of material. Vacuum evaporations are also available; some kind of physical vapor deposition and Chemical vapor deposition. There are other methods uses for more sensitive Nano membrane, now we consider organic ones, these type or methods are usually applied for more enduring ones. Another approach is using spin coating. One of the convenient approaches is to use electrochemical deposition for sensitive multi-layers. There are others micro fabrication techniques use for such material which include chemical deposition, dip coating and others.
2. Langmuir- Blodget: For the fabrication of lamellar Nano films, this technique is use by using phospholipids, fatty acids, glycolypids, and amphiphilic molecules. It is laminated by the interface of water/air including such molecules with Nano membrane; mono molecular film formed over it. To get the desired lamellar multilayer the procedure follows again and again. The figure below shows the process of lamination when substance is put in a mixture of laminated material and water and it attached with the substance like this. Left side of figure is showing the Langmuir technique for hydropholic while the right side of figure is showing the Lanqmuir technique for hydrophobic. To get the high quality result, process repeat until we get a required quality .Every time we will get a good result than previous one,some time it is supposed that; after first lamination it will be bad to laminate again, So it is clear from now this concept is totally wrong and we can laminate a substance more than one time.
Figure 5: Langmuir Blodget technique
a) hydropholic
3. Techniques Layer-by-layer: This is a techniques use for lamination process; it uses the macro molecules of oppositely charge to alternate adsorption. In fig, it represents the first diagram is of first layer Polyanion , the middle layer is of Polycation, and the last diagram is final structure of composite nano membrane. The substrate of silicon is chemically react with the cationic polyelectrolyte dilute solution . The substrate absorb polyanion and make a single layer over substrate of 1mm thickess and its all depend on the material use for the this. The water of a diluted solution is the dried after rinsed. The layer of substrate covered with polycation Is the plased in the solution of diluted polyanion nano particles for new layer on the previously deposited layer on the surface. The water of substrate is again and again dried after rinsing. This is the single cycle for making layer by layer lamination. To increase the quality of lamination number of cycle increase and more and more layer can be laminated over substrate.
Figure: Composite Nano membrane
Filtration is the process in which liquid or gas is passed by some substance to separate particles and to get the required particles of gas or liquid. This technique came from very basic real life application; to separate heavy particles from small particles by using net or something like that. We define a pore size to allow small particles and restrain big particles of a liquid or solid. Filtration using nano membranes have same principle. To select the degree of membrane pore size of membrane is considered. The pore size defines the allowable and restriction limit of a substance which is needed to be filter. It all depends on the pore size, which classifies its character. The nano membranes filtrations have four types. It can be classified as:
a) Macro Filtration
b) Ultra Filtration
c) Reverse osmosis
d) Nano Filtration
Macro Filtration:
To separate macro particles of a filtering substance this macro filtration technology is use. In nano membranes filtration; pressure is also need to pass substance through nano membranes to filter .It is use to remove particles higher than 0.08 ‘ 2 um so the nano membrane is fabricated like this to allow higher particles of this thickness by setting the value of pores and pressure is required for this is comparatively low than the other filtration method; it operates within a range of 7 ‘ 100 kPa pressure. The main working of micro filtration is to remove residual suspended solids (SS), to remove bacteria’s from water in order to make it dis-infective and effective for drinking purpose and also use as a pre-treatment of reverse osmosis.
Ultra Filtration:
Ultra filtration is use to separate small particles comparable to macro filtration. Its pore size is also small as compared to macro filter. It is usually use to remove particles higher than 0.005 ‘ 2um and it also required some pressure to pass substance through membranes. The pressure on which it operates is within a range of 70 ‘ 700 kPa. As both micro filtration and ultra filtration are both used for filter purpose so many applications are same but ultra filtration is also used to separate dissolved compounds with higher molecules in a liquid like carbohydrates and protein etc. it is also use to remove toxic material and viruses from liquid.
Reverse Osmosis:
This is another type of filtration. Its pore size is smaller than macro and also ultra filtration pore size that’s why it is use to filtrate or separate very small particles from liquid or a substance which is need to purify. It is basically a water purification technology and uses semi permeable membrane. Its thickness is greater than Nano but less than ultra and macro membrane. Usually for water purification, substance is first pass through macro filtration to separate heavy particles from liquid and then pass the liquid from reverse osmosis process for more purification of liquid, the benefit of doing this procedure is; large particles are already separated from liquid and in reverse osmosis only small particles separate otherwise, heavy particles will also separate and will require more cleaning of membrane and process will slow. The pressure required in reverse osmosis is very high than the both ultra and macro filtration pressure to pass liquid. Its pressure goes from hundreds of kilo Pascal to thousands of kilo Pascal and it required a high pressure of 850 ‘ 7000 kPa for its process. It excludes ions from water.
Nano Filtration:
Now see the nano technology of membranes. Nano filtration is a nano technology of membrane for filtration. This type of filtration is use to separate nano size particles from liquid. It is able to remove very small particles of liquid i.e. smaller than 0.001um. It is basically used to remove selective particles from waste water; those particles are dissolved in it. It was made for softening purpose of membrane that offers an alternative to softening through chemicals. Nano membrane is also becoming more important to used in the applications of food processing such as water purification, diary etc. Its pore size is smaller than MF and UF but larger than RO pore size. Commonly used materials are polyethylene terephthalate or metal like aluminum. It pore dimensions are controlled by using pH, temperature and time in development with pore densities range 1 ‘ 106 pores per cm2 .
Recent Development In Nano Membrane:
Now a days, Nano membrane are also using in electronic devices which are usually called semi conductor Nano membrane, this semi conductor technology has very thin layer of single crystal sheet of ten Nano meters. For new science and novel devices; it offers many opportunities. Epitaxy involves introducing strain. By combining epitaxy with Ge and Si Nano membrane; it can be achieve advances in group IV photonics, thermoelectric, and optoelectronics. New strained formations are allowed by using Nano epitaxy. It also allows lattices of periodic strain and hybrid composition or orientation with membranes.
Semi conductor technology use alternative bilk substrate like crystalline Nano membranes and it may make a new application of Group IV material especially. Bulk material and Nano membrane are usually distinguished by their flexibility, thinness, stack ability, shapes conformability, easy bond ability and the shorter distance between two surfaces. To achieve higher level of control; the devices based on Nano membrane structure usually combine bottom up and top down approach, it can be shaped in 3D structure , flat and may be completely free standing to a substrate. Main focus is being given on Group IV elements and the reason is Si forms the base of our communication and computing. Also use in new technology and energy conversion.
When do we talk about Si Nano membrane; first thing comes in our mind that there are many application of si Nano membrane and have already worked a lot in it; that are based on the ability in processing Si Nano membrane like a Si bulk also there is very flexibility in it. To combine epitaxy with Nano membrane, there is a need to make it thin and mechanically stable or good; it is a very interesting effort putting on it. Epitaxy can be use to make a spatially or uniform strain field in crystal sheet of ultra thin thickness. The possibility of changing properties of material like carrier mobility and band gap is offered by Strain engineering of Si Nano membrane. To the fabrication of novel electron and high performance structure; it has capability to develop membrane based technology for it.
Our focus is on recent development of epitaxy, to find out or make a new properties element and new materials. To create hybrid composition or hybrid orientation, crystalline and double sided QD epitaxy by using the bonding and layering of membrane and epitaxy.
Researchers are doing research to optimize the process of nano filtration and also trying to improve the nano membranes performance. The main problem in the nano membranes during process of nano filtration is fouling and there is a need to minimize the fouling of membrane, so it is also in under consideration. The energy is required in the process of nano filtration; reduction in energy is also in research. The performance of nano membrane can be improved by lowering the resistance of membrane and increasing the flux permeate; it can be done after doing many experiments with different configuration, condition and material. The most commercial success of nano membrane in the industrial area is thin film composite TFC membrane that consist of many layers of thin film of thickness in nano size; which are usually selective layers on the substrate. It is discussed in next paragraph.
Strain and Epiaxy
In the growth of making a thin film layer on bulk substrate; strain in epitaxy role is very common. In composition process, the film gets thicker and thicker, so the energy of strain increases at a critical level of elastic limit and due to dislocation formation the film get relax. There are both thickness kinetic critical and thermodynamic thickness. By one or two competing mechanism a strained film can relax because of the formation of 3D islands of coherent nanostressor or due to formation dislocation. When the strain is relatively small the strain energy tends to release plastically by dislocations formation.
Figure: this graph shows kinetic critical and thermodynamic critical thickness on Si of strained SiGe.
In the above figure the x-axis is representing the Ge concentration while y-axis represents the critical thickness and its unit is in Nano meter (nm), the range is defined in the figure is; 1 nm to 3000 nm thickness. There are three states in a figure, one is stable (Mathew Blaeslee), and second one is meta-stable while the third one is relaxed state. The above figure basically shows the strained SiGe film on Si, the thickness of thermodynamic and kinetic are very critical. For a typical growth of 550C temperature the effect of critical kinetic thickness is shown. As you can observe from figure, on increasing the concentration of Ge the thickness is decreasing. The epitaxial relationship of strained 3D Nano cluster with the substrate when they dislocate at the interference due to becoming too large; strain relief is provide which allow epitaxillay growth of more material without dislocation formation than the 2D allowable critical thickness.
.The larger misfit films strain like Ge on Si use 3D formation of island after the exceeding of the 2D film thickness.
The 3D technology of nano stressor has been discussed in detail. The thickness of wetted layer is controlled by the degree of strain and also the composition material and its all can be accomplish by substrate of Si by expansion or curvature of the substrate.
Left side of above figure is showing the single ridge while the right side of figure is showing the cross of two ridges .It is a 3D image of Nano stressor epitaxy and higher curvature ordering region. The red and green line in above figure is the indication of periodicity qualitative of Nano stressor. The above figure shows the Nano stressor of Ge islands increase where the curvature is highest and in ordered patterns. The inherent curvature and wetting layer of the function of strain term is usually known a nucleation. In the first Ge deposited layer; the strain is noticeable. The deposition on vicinal Si of Ge made its influence on the structure step as a result. As the composition increases thickness also increase, so the addition of Ge allows expansion that is overcome by using strain and on adding more Ge causes dimmer reconstruction induced extraction of surface stress for clean Si.
Artificial Nano membranes:
The structure of biological material are organized hierarchically, it means the material start from very small block and gradually make a huge material or a unit. From simple building blocks make an organized complex unit which is usually more powerful or it will be better to say efficient than a basic block because it may now contains qualities of different blocks. So it makes a series of new set of building block for higher toolbox hierarchy.
Same like Nano membranes, they are actually on low architectural life and it can make a big unit if works on it to get a better material or a huge block. It is like a first brick of pyramid and by giving effort on it will make a huge pyramid. Now comes to the biology, it means Nano structure; because every cell and a part of cell are all Nano sized. It makes a huge body by simple cell is a Nano size. If we talk about virus, bacteria and every prion are all Nano sized structure. The building blocks of living organism are mostly represented as sophisticated 3D Nano system because every small particle is in Nano size.
The Nano membrane is practically unavoidable to take Nano structural as a unit in the path toward the top of hierarchical pyramid of life. Usually living cell that enable the functionality of the cell are lipid bilayer membrane with various. Actually, it activate its interaction with cytoplasm and divides the cytoplasm of the living cell from its environment. The metabolic processes proceed through Nano membrane in each cell and also its active participation. In a life form we can find a many varieties of Nano membrane and its existence is enabled through Nano membranes; it is because Nano membranes are the most building element of life. And as we discussed before, we can find applications of Nano membrane in a very small particles which serves as a building block for them like bacteria and virus etc. So, all biology is based on Nano particles and after making and joining small particles made a large hierarchy and an organized structure.
After discussing the importance of Nano membrane, now we can also look around the living thing of the world with an eye of membranes technology. Now a day, their development is proceeding in two different directions. Using brute force is the first approach; in which manipulate molecules and atoms during device fabrication. It is basically an oldest technology; during the process of fabrication of device atoms and molecules are replaced by using force. Now here we give a simple example to understand the importance of Nano membrane, everyone knows about the memory chips and microprocessor; if we see the development of microprocessor and memory chip we would find that a small transistor was behind its development. By combining a large number of transistors in tens and hundreds million and trillions we were able to make a microprocessor. So from here we can conclude that a large packaging of small and simple element we can make a very interesting and useful thing we cannot imagine now, just we need very precise alignment tools, cleaner rooms and a process control of increasingly accurate. Using a simple behavior of Nano membrane, we are able to make a large quantity with a great use like by using a basic behavior of transistor made a microprocessor.
In the improvement of quality of life micro technologies contribute very much without any doubt. The explanation is given in above paragraph; the importance of micro particles and its technology help to make new developments in the field of science and technology. It was not possible in the history to interchange and safe the information of record. Without the result one cannot get the whole field o human activities. So to understanding the technology and process one should have information, moreover the practically result from this; one could decide and make up their mind and after this one could be able to make a variety of successful development which are desired.
Nano membranes technology is already worked on making artificial structures of membranes and still working on it. By using nano membranes technology; new structure may be introduce which may not existed before. The benefit of artificial membrane is; we can make our required membrane and everything will be in our hands to select different options. Nano membranes technology gives us very easy environment in making a new structure membranes and offered artificial structures. It gives us a chance to define membranes functionality of our demand, and then working on it will give us more and good results. By putting effort in the development of membranes of basic block will extend these nano membranes in organic and also in inorganic functionalities.
Inorganic membranes:
Inorganic nano membranes are newer than the organic nano membranes but it is very simple I structure than tha organic membranes. When we go in the history of nano membranes then we would be able to find that organic nano membranes have very old history while inorganic nano membranes have recent history of about hundred years newer than even artificial organic nano membranes. Inorganic nano membranes are made only because of fabrication procedure of nano membranes by reliable and repeatable procedure of making nano structures.
The structure of inorganic nano membranes is simplest in all nano membranes. Its properties show its functions of complex organic nano membranes. By studying the properties of inorganic NM; we shall be able to find the function of NM in the process. So, to get the required function from NM we just need to know the properties of inorganic NM and we can make our new own function. There are many structures of inorganic NM on their composition known as freestanding structures. It defines in different classes.
a) The one and very simplest class use nano membranes of pure element; it is further divided in three sub categories.
1. The first subclass of pure element nano membranes is the structure of pure metal.
2. The second subclass of pure element nano membranes is carbon based nano membranes. In which diamondoid and diamond are the most important ones.
3. The third subclass of pure element nano membranes; include the structure of semi conductor element membranes. The most important element is silicon structure. It has a very dominants position in the field of nano membranes. After doing research, we got a result of using silicon in the field of micro system is about 90% and above
b) There is another class which is also very vast includes inorganic compound of simple elements like nitride, oxide, carbides and other material. In these oxides SiO2 has a very major contribution because of its dominance in membranes and it takes a central position in oxides.
c) Now comes to the third class of inorganic structure. It includes ceramic and glass nano membranes. It is very important class in the field of inorganic nano membranes structure.
Nano Membranes of Pure Metal:
Pure metals nano membranes are very simple in structure. Nano membranes that are consisting of pure metal are considering simplest nano membranes. The material use in pure nano membranes are nickel, chromium, palladium, aluminum, silver, platinum, gold and many other elements like that. These are usually use to strengthen the property of nano membranes but some metals have also worse mechanical strength; so these type of metals are use for other purposes. The pure metal nano membranes have many applications in micro electronics and other are usually known as a catalyst; because pure metal nano membranes are also use in catalysis process. There are some other metals uses in Nano membranes that are tungsten, copper, lead, titanium and tin. Pure metal nano membranes are usually chemical inert and do not react with the substance which is required to filter of separation. The first pure NM was made in 1931 and it had 80 nm in thickness. It was the great achievement at that time when this membrane was discovered by Winch manufacturer. The process used for the manufacturing of this membrane; polished halite material surface of nano membranes was sputtering with gold; first the membrane was polished with sodium chloride to improve its property then to make metal nano membrane it was sputtered with gold. When the gold film deposited over nano membrane the halite substrate i.e sodium chloride dissolved in water and left the nano membrane surface; it was then floating on the water surface. In 20 century, there was not much worked on nano membranes of metal. Now we are taking it very beneficial for future, so many scientists are working on it now for new metallic membranes. In the time of 20th century, metallic nano membranes were made for specific applications. It was not using for commercial purposes that why there was not very high research over it but a few research to fulfill basic requirements. Lets discuss about the first metallic nano membranes area; its area was smaller than 0.1mm2 , it is very small area for the first one but now there are much smaller metallic nano membranes are developed than this area. The aspect ratio of first metallic NM was below 500. The property of that NM was fragile and brittle. It was brittle in nature may be because of metallic property; usually metals are brittle in nature. The most interesting property of metal is; they are electrical conductive so the metallic NM are also electrical conductive in nature and it can conduct current through itself. To fabricate ultra thin layer of pure metallic NM; use micro system technologies which are use conventionally. The very famous micro system sacrificial supporting structure technique was used, the techniques was defined by Fauchet and Striemer. In this technique, the surface area of pure metal NM up to 10 mm2 was obtained 430000:1 aspect ratio.
Modifications in Structure:
The structure of nano membranes can be modified for different function of our requirement. The structure defines the function of nano membranes and based on patterning structure of NM. The different pattern gives different functionality. The structural geometry of nano membrane can be controlled by using the modification and making an array of nano holes or nano apertures in 2D. The fabrication of array in 2d nano holes also known as nano pores are made by using nano patterning of NM. This 2D array has its own functionalities. It is basically belongs to the patterning of nano membrane. The figure shown below shows the modification in structure of nano membrane in 2D array. Micro contact printing may also use for nano patterning of nano membranes; there are many approaches of micro contact printing and the basic one is dry lift off approach while the others approaches are also use for micro contact printing. Nano membranes are very small; its pore size is in nano size and the thickness is also in nano size and because of very small thickness and pore size the ratio between them remains small. The small ratio of thickness and pore size makes fabrication very simple and the control of dimensions of pore during fabrication of nano as well as micro membrane has become much easier.
The above figure shows the structural modification of metallic nano membrane by using 2D array pattern of nano aperture. From figure, it can be seen that the left one is elliptical type of pore and it has an opening of elliptical for the substance pass through it. It is fabricated in 2D design of nano pattern. The right side of figure shown the fabrication of nano membranes is structured as circular type of pore for filtrating material to be passed through it. The dimension is defined by their use in the process. Figure shows the array of nano patterning and gives different functionality.
Striemer described the method of making pores in silicon NM. The technique was to deposit silica with silicon and the other side is also silica so that it makes a sandwich of silica silicon silica onto sacrificial substrate of Silicon. After annealing the nano membrane of Si the pores are formed over it. Now let see how can we modified the pore size of membranes, its all depend on the annealing. When the temperature is higher in the process of annealing the pore size we get will be larger in size and when at the lower temperature is set in the process of annealing then the pore size will appear small. To get a required pore size, we just need to make a graph of pore size with respect to the annealing temperature, so that by setting the annealing temperature pore size can b define by manufacturer.
The above figure is showing the procedure of making silicone nano membrane. There are basically four figures step by step process of fabricating Si nano membranes pore. The top left figure show the sacrificial silicon substrate which is represented by sky blue color. The top right figure is showing the deposition of sandwich on SI substrate. The deposition on Si substrate is in sequence of silica or in other words silicon dioxide, the next layer is silicon and the third layer is again silica – silicon dioxide SiO2 . Now let come to the left bottom figure, it is showing the formation of pores in Si substrate after annealing process in temperature. As discussed before, its pore size can be vary with temperature and to get higher pore size annealing temperature should be kept high. Right bottom figure is showing only the pores and when see it precisely, you will observe the slitting of pores, and it is basically the slits on silicon substrate. The blue lines are showing the Si substrate while the off white line is representing an opening or pores.
The above figure is showing the different modifications of nano membrane. These are surface type of modification. It can be observed that the various type of modification can be formed by using surface modification of nano membrane. The selection of modification is defined by the function where it has to be use and on the requirement of use. If we see these figure with an eye of lay man who does not know about the nano membrane, he can also understand the different structure and its use, it works like a grill. Let see the bottom figure, the left side can be use for the filtration of small particles; it can pass small particles while block the larger particles greater than its pore size and the right side can be use to separate particles but larger in size than the left one because its pore size is comparatively large than the previous one and it is able to pass large particles through it.
Modification on the Surface of Nano membrane:
We have discussed the surface modification of nano membranes structure. There are sub categories on which structural modification can be done. It includes number of sub classes and have very diverse and rich group. Let’s discuss the few of them;
1. The first class is nano patterning. In nano patterning the structure of nano membrane is made by deposition of different chemical layer on the nano membrane surface and make a pore size in the 3D structure. The pattern of nano membrane can modified the optical and electromagnetic behavior of the surface; the pattern defines the property of nano membranes e.g the strength of electromagnetic of nano membrane. The pattern also cause the reason to improve the nano membranes repulsion or attraction with the living agents or biological agents; on the requirement of function we can make the property of nano membrane either the attraction of biological agents to the nano membrane or repulsion of biological agents to nano membranes.
2. Through deposition process chemical activation of nano membrane can be taken place. Also the biological and bio chemical activation can be accomplished by using deposition e.g catalysts, ligands etc.
3. The passivation treatment of surface can be done by using structural modification, it is necessary for chemically or mechanically unstable nano membranes. It is basically a surface protection treatment to safe nano membranes from oxidation. Chemically unstable nano membranes mean it is highly reactive to outside chemicals, and it can react to make a chemical layer over it; usually oxidation like metal got rusted because of oxidation over metal and metal got mechanically weak. Same like nano membranes after oxidation due to chemical unstable; become mechanically weak, so it is very necessary to do surface treatment to make it chemically stable.
From the previous topic; the figure shown above is the application of nanolithography for the surface modification. The modification in a nano membrane can be done by itself or it can doing by using deposition; the additional pattern deposition on the nano membrane. It can be assume the deposition of stripes and pillars in various forms. The benefit of additional pattern is to make a nano membrane with multi pattern and increase the quality of nano membrane, after doing that nano membrane can work more efficient than before.
There are many applications of Nano membrane. Some of them are:
1. Energy Conversion and Production: The application of Nano membrane in energy conversion and production is Fuel separation or purification of fuel from its unwanted particles and elements, it can also be define as fuel is actually in raw form in earth but to use for our self we need to separate in different categories so Nano membranes help us a lot in separation of fuel and if there are few contents of other elements it also provide us to purify the fuel by removing of selective particles from the fuel; helps in hydrogen extraction from raw fuel and make it pure and useful. In the production and purification of nuclear fuel Nano membranes are use for this purpose because Nano membrane are very good in selective separation and we need a very high precision and accurate separation in nuclear fuel and we can only trust on Nano membrane for this separation, hydrocarbon fractionation, production of fuel for environment friendly.
2. Treatment of Waste water: There are many contaminants present in water like natural organics matter and some traces of accumulating organic in water. There are many processes for the filtration of waste water and to get a drinking or a soft water. The processes are chlorination and flocculation that has been used for softening and to remove contaminants of water. There are few problem arise in conventional type of filtration. These types of technologies are unable to remove contaminants completely but it makes additional sludge in water which creates another problem and we don’t want this type of sludge because it is also a contaminant for water. So to avoid this sludge we must use Nano membrane because Nano membrane do not cause sludge problem in treatment of water. When use Nano membrane for filtration it removes particles completely in less producing cost and makes it effective for commercial use. The above reasons force us to use Nano membrane technology rather than conventional chlorination and flocculation technology.
3. Biomedicine: There are many applications of Nano membrane in Biomedicine. It is used in the treatment of cancer, blood, and in drug delivery system. Also in diagnostic instrument, medical sensor devices, immune system navigation and health care system. Many biomedicine instruments are made of Nano membranes so Nano membrane has importance in biomedicine.
4. Chemical Industry: While discussing about the usage of Nano membranes in chemical industries. We will find many surprising applications in this field of chemical industry, It is used in manufacturing process, food and beverage processing, Nano membrane use in separation of multi components of gas mixtures to make it purify and sort out the required gas without the contaminants particles; which may be required in some process of industry, Nano membrane are also use in sea water desalination; there are many salts in a sea water which makes it hard water and to use water we must make it soft water; for this purpose Nano membranes separate that salt particles from hard sea water, dehydration of gas and many other separation of different chemicals; zeolite material is mainly use for industrial purpose. Also use in microscopic structure of ceramic membranes of nanoporous. Nano membranes required more advancement to use in many things and in more efficient manner. There is a need of more and more membrane separation technology to fulfill requirement of developed chemical synthesis process. After making new technologies of Nano membrane we will be able to separate more and more unwanted particles and make our substance more purified.
5. Bio Engineering: Nano membranes technology not ends on biomedicine. It also has an application in Bio engineering. It is use in analysis of DNA, bio technology and cell biology. For analyses purpose each particle need to separate for study and observation and Nano membrane do a good job. And Nano membranes are also use for analysis by reacting component with it and analyze by observing results, so Nano membranes have diverse usage and importance in bio engineering .Also use in bio chemical sensing, virus analysis, protein bio marker and prion analysis.
6. Protection of Environment: In the protection of environment Nano membranes have a very important role. Air stream have many organic particles in it and Nano membranes use to separate that particles. It is use to recover organic components from air stream, separate valuable chemical from liquid so that to safe chemical by restraining through Nano membranes and pass all other liquid through Nano membrane, treatment of waste water, production of potable water and purification and recycling of water as discussed above in detail.
7. Food Industry: Nano membranes have many usage and applications in food industry. Main purpose of Nano membrane is separation, so it is use for separation process in food industry as well. It is use in food processing and has many advantages in separation. Now a day, there is a new technology of Nano membrane, which is Nano filtration; is widely used in food processing; it is placed between ultra filtration and reverse osmosis. So far we have discussed Nano filtration again and again because it is the new and main technology of Nano membrane that is widely being use in many fields.
8. Medical Application: Now comes to the medical application and sees the application of Nano membrane in this field. It is use in many applications of medical, various kind of application can be find in a Nano science technology where Nanoporous anodic aluminum oxide (AAO); because of its bio compatibility has become very important.
9. Bio Technology: It is use in many bio technologies and has future application of Nano membrane in this field.
10. Toxicology: It is use to recognition of organic, biological substances and harmful inorganic components.
11. Forensics: Nano membranes are use in forensic purpose to test the particles and to find the material recognition. Nano membranes are also use in forensic to separate particles of tested material to find out and to check the material from different scenarios.
12. Homeland defense: Nano membranes are use in home land defense in finding the harmful organic material from a substance. It is also use to find the presence of biological agents and inorganic in a substance which can be harmful for it.
Advantages and Disadvantage:
The main advantage of using Nano membrane over conventional method for filtration and other purposes is that; during its process the magnesium and calcium ions that are retained does not required replacement of sodium ion for exchanger. In conventional method of filtration, during process magnesium and calcium required a replacement of any other element to compensate, so sodium ions are used to exchange it. The disadvantage is; many process of separation that does not operate at room temperature like distillation; it causes a large increase in cost in continuously heating and cooling the temperature of process. It also required pressure for its operation and optimized condition for its process otherwise it causes damage of nano membrane. One and the main disadvantage with membranes are the cost and its maintenance. Nano membranes required cleanness and even change after use of some time. If the operating conditions vary from the specified condition, nano membranes start distortion and need to change after few processes. Also does not give good result than starting or fresh membrane, its efficiency decreases with time. In the process of Nano membrane filters; Nano membranes are expensive which make its whole cost expensive. The total dissolved solids defines repair and replacement of membrane; also the components of feed and flow rate are necessary to be consider for repair and replacement.
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